Contributions to MDN are welcome. All you need is an account, and their reference reads more like documentation and less like an ad :)
You can even log-in via GitHub nowadays.
The remaing space is 150px
Also, I lead a fairly ad-free life, so there's way too many ads for me to consider using it regularly - those fixed-on-the-screen ones are especially intrusive. A shame as it looks like a good idea.